As an optical head which writes optical information or reads optical information on an optical disc, a magneto-optical disc and so on, there have been known one wherein a prism type beam splitter is used as an optical part for directing (beam-splitting) a signal ray reflected from a recording surface of a disc to a detector, and one wherein a diffracting grating or a hologram element is used as such an optical part.
In the past, the diffraction grating or the hologram element for the optical head has had a rectangular grating with a rectangular section (an isotropic diffracting grating in a relief form) formed on a glass substrate or a plastic substrate by dry etching or injecting molding, thereby reflecting a ray to obtain a beam splitting function.
In order to increase the utilization efficiency of a ray in comparison with an isotropic diffraction grating having about 10% of light ray utilization efficiency, the utilization of polarization can be thought out When polarized light is utilized, there has been a method wherein a prism type beam splitter is combined with a quarter-wave plate to improve efficiency in a forward path (a direction from a light source toward an optical recording medium) and in a backward path (a direction from the optical recording medium toward the light source and a detector) so as to raise reciprocation efficiency.
However, the prism type polarization splitter is expensive, and another system has been groped for. As a system, there has been known a method wherein a flat plate of birefringent crystal such as LiNbO.sub.3 is used and the flat plate has an anisotropic diffraction grating formed thereon to obtain polarization selectivity. However, such a birefringent crystal per se is expensive, and application of the birefringent crystal to commercial utilization is difficult. When the grating is formed by proton exchanging, the proton in the proton exchange liquid is usually liable to diffuse in a LiNbO.sub.3 substrate, creating a problem in that it is difficult to provide the grating with a fine pitch.
The isotopic diffraction grating has about 10% of reciprocation efficiency at the maximum because the utilization efficiency in the forward path is about 50% and the utilization efficiency in the backward path is about 20% as stated earlier.
The present invention has been made to solve these problems, and has an object to provide an optical head having high utilization efficiency of light ray and high reliability.